The need often arises to secure objects such as paintings, fittings, decorative articles and the like to vertical surfaces such as walls. Many securing devices have been proposed to satisfy this demand, with a common requirement of such devices being sufficient and reliable load bearing capacity.
A typical prior art securing device includes a hook or other engagement formation that extends from a base, the base being securable to the supporting surface on which the securing device is mounted. The base and hook is often integrally formed, but the hook may also be attached to a flexible adhesive-bearing sheet. An object as described above is then suspended from the hook, thus imparting a downward gravitational force on the hook. The point of contact between the hook and the suspended object is usually spaced apart from the base, and more particularly from a securing surface of the base. The distance between the securing surface of the base and the point where the downward force is imparted results in a moment being induced about the securing surface of the base. The moment about the base translates into forces, perpendicular to the supporting surface, being exerted on the base. At the operatively upper end of the base the force is directed away from the supporting surface, thus constituting a tensile force, whereas the force at the operatively lower end of the base is directed towards the supporting surface, thus constituting a compressive force.
In many instances a load applied to the hook or engagement formation is not parallel relative to the base of the securing device, and may for instance be directed at an angle, away from the supporting surface. In this case the exerted force will be in the form of a force vector having both vertical and horizontal components. The horizontal component of this force vector will then also contribute to the tensile forces exerted on the base.
It will be appreciated that due to the above configurations, an adhesive medium used to secure the base to the supporting surface, and more particularly an adhesive film located on the adhesive medium, is exposed to tensile stress due to the tensile force at the upper end of the base, as well as shear stress due to the gravitational force. This combination of forces adversely affects the load capacity of the adhesive medium, which is substantially lower than it would be it the adhesive medium were exposed to pure tension or pure shear.
Some solutions have been proposed to alleviate this problem, as is for instance disclosed in Dutch patent NL1028204 in the name of Peer Schoofs te Gemert (“Gemert Patent”). The Gemert patent shows a securing device having a first securing member in the form of a pin to be inserted into a wall, or into a gap between adjacent tiles, and a second securing member in the form of an adhesive member. The aim of the securing device is to prevent the adhesive member from being loaded in shear, but in order to achieve the same the pin must either be secured in the wall, or located in an aperture provided in the wall. It will be appreciated that it is often not desirable and/or practical to make an aperture in a wall or other supporting surface. Furthermore, existing apertures, such as the gap between adjacent tiles, usually do not exist. Also, if the pin is secured in the wall as envisaged in the Gemert patent, a moment and large local forces will be induced in the wall surface about the pin, which may very likely damage the wall surface. In use, the outcome of resultant forces on the two securing members is quite unpredictable, depending entirely on the nature of the securing surface. Similar disadvantages are foreseen insofar as the securing devices disclosed in CH670366 Freimann and GB2373287 Story are concerned.
Many other securing devices, including those disclosed in DE8625361 Pagenberg, DE9108687 Westphal, DP29821567 Pagenberg and U.S. Pat. No. 2,724,568 Rabinovitch, have been proposed wherein the securing device includes more than one adhesive securing member. However, in none of these cases is the securing device configured for tensile and shear forces to be separated, in order for each securing member only to be exposed to tensile or shear stresses.
The problem of reduced load capacity is further exacerbated by the presence of peel loading and cleavage loading, which are described, and for the purposes of this specification defined, below.
Peel loading typically occurs when the following conditions apply:                The adhesive medium is carried by a base that is relatively flexible in bending;        The base is subjected to a tensile force in a direction away from the surface to which it is bonded, i.e. there is a force component normal to the surface;        The backing is sufficiently flexible in bending to bend under the tensile force; and        The base is stronger in tension than the peel strength of the adhesive bond.        
Under these conditions, the energy transmitted by the base as a result of the tensile force is focussed on a line of high stress at the location where the backing is separating from the surface. The applied force is focussed on this single bonding line, thus resulting in high-energy concentration, and adhesive failure in a progressive manner.
Cleavage is somewhat similar to peel, but occurs when the base is relatively rigid, so as not to bend or flex perceptibly under the applied loading. However, a tensile force component normal to the surface is still present, and cleavage occurs when this tensile force is not uniformly distributed over the bond area under the backing. Moreover, cleavage loading situations are characterised in that the backing is usually not completely parallel to the surface to which it is bonded, or it is not constrained to remain parallel. As a consequence of the non-uniform distribution of the pulling force or the non-parallel orientation or both, there will be places where the tensile stress in the adhesive bond peaks at its highest value. If the tensile stress exceeds the local adhesive bond strength at any point, the adhesive bond may fall at that point and run in a progressive manner through the entire adhesive bond under the rigid backing. In structures under static loading, once localised failure in the adhesive bond has started, the stress in the remaining adhesive bond increases, since progressively less bonded area remains to bear the load. Cleavage failure therefore typically starts off slowly and increases in tempo as failure progresses.
It is clear from the above that cleavage and peel loading are undesirable where a durable adhesive bond is required. For maximum strength, cleavage and peel loading should thus be avoided as far as possible. If there is any possibility that a peel or cleavage process may occur in a product where adhesive bond strength is important, such a product may be prone to premature failure.
In addition to the need for high load bearing capacity, it is also preferable for a securing device to be easily removable from a surface on which it has been mounted, without damaging such surface. The well-known standard double-sided tape, comprising a backing material having adhesive films on opposing surfaces thereof, is often used as an adhesive medium for securing devices. However, removal of standard double-sided tape often proves to be troublesome because the backing material of the double-sided tape, usually comprising an elastomeric foam, tends to tear or break, thus preventing the double-sided tape from being removed as an intact section of tape. In addition, the bond between the double-sided tape and the supporting surface is often strong enough to result in the supporting surface being damaged when the securing device mounted by double-sided tape is pulled from the supporting surface.
Several patents, including U.S. Pat. No. 5,409,189 Luhmann, U.S. Pat. No. 5,984,247 Luhmann, U.S. Pat. No. 5,989,708 Kreckel and U.S. Pat. No. 6,001,471 Bries, disclose the use of so-called stretch release doublesided tape in providing removable securing devices. Stretch release double-sided tape is a special type of double-sided tape, and is commercially available from companies such as Beiersdorf AG and 3M. Stretch release double sided tape involves the progressive, controlled destruction of the adhesive bond on both adhesive sides of the double-sided tape when the tape is firmly pulled at one end of the tape along its length. As the tape material stretches, the adhesive bond is broken in progressive manner until the tape is fully stretched and the adhesive bond completely broken, thus releasing the securing device from the wall.
However, stretch release tapes are beset with numerous problems, which are inter alia documented in the above patent specifications. A first problem is that the backing material may tear before the adhesive bond is completely broken. When the backing material, and thus the tape, tears, it becomes almost impossible to remove the securing device without damaging the surface on which it is mounted. Ageing of the tape increases the risk of tearing, as well as excessive pressure applied by a user onto the securing device as the user holds it during removal. A further problem is that the tape may exhibit a substantial amount of recoil when the securing device is released. This recoil action may easily cause physical injury to a person trying to remove the securing device.
As described hereinbefore, some securing devices utilise an adhesive-bearing sheet that bonds the securing device to the supporting surface. The adhesive-bearing sheet typically comprises some sort of backing material having an adhesive film on at least one surface thereof. In use the adhesive-bearing sheet is, amongst others, subjected to a force component substantially parallel to the supporting surface and the adhesive-bearing sheet so as to result in a shear loading, and thus shear stress, in the adhesive bond between the adhesive-bearing sheet and the supporting surface. One problem associated with existing adhesive-bearing sheets is that the backing material is often not sufficiently rigid to ensure uniform distribution of the shear loading over the entire adhesive film, which may result in excessive stretching of the backing material, which causes non-uniform loading and thus premature failure of the adhesive bonds.